Recent advances in communications technology have enabled an emerging, revolutionary ultra-wideband technology (UWB) called impulse radio communications systems (hereinafter called impulse radio).
Impulse radio has been described in a series of patents, including the following, which are incorporated herein by reference: U.S. Pat. No. 4,641,317, issued Feb. 3, 1987 to Larry W. Fullerton; U.S. Pat. No. 4,813,057, issued Mar. 14, 1989 to Larry W. Fullerton; U.S. Pat. No. 4,979,186, issued Dec. 18, 1990 to Larry W. Fullerton; and U.S. Pat. No. 5,363,108, issued Nov. 8, 1994 to Larry W. Fullerton. A second generation of impulse radio patents include the following, which are incorporated herein by reference: U.S. Pat. No. 5,677,927, issued Oct. 14, 1997 to Fullerton et al.; U.S. Pat. No. 5,687,169 issued Nov. 11, 1997 to Fullerton et al.; and U.S. Pat. No. 5,832,035, issued Nov. 3, 1998 to Fullerton et al.
Basic impulse radio transmitters emit short pulses approaching a Gaussian monocycle with tightly controlled pulse-to-pulse intervals. Impulse radio systems typically use pulse position modulation, which is a form of time modulation where the value of each instantaneous sample of a modulating signal is caused to modulate the position of a pulse in time.
For impulse radio communications, the pulse-to-pulse interval may be varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random code component. Unlike direct sequence spread spectrum systems, the pseudo-random code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the pseudo-random code of an impulse radio system is used for channelization, energy smoothing in the frequency domain, and interference suppression.
Generally speaking, an impulse radio receiver is a direct conversion receiver with a cross-correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The data rate of the impulse radio transmission is typically a fraction of the periodic timing signal used as a time base. Because each data bit modulates the time position of many pulses of the periodic timing signal, this yields a modulated, coded timing signal that comprises a train of identically shaped pulses for each single data bit. As an option, the impulse radio receiver can integrate multiple pulses to recover the transmitted information.
In a multi-user environment, impulse radio depends, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high processing gain achievable with impulse radio, much higher dynamic ranges are possible than are commonly achieved with other spread spectrum methods.
Radio frequency (RF) applications for positioning, locating and tracking are widely known. The Global Positioning System (GPS) is a popular example of an RF positioning application. Some other examples include RF tags, military air combat maneuvering indicators (ACMI) and asset and people tracking devices.
For example, commonly-owned U.S. Pat. No. 6,111,536, xe2x80x9cSystem and Method for Distance Measurement by Inphase and Quadrature Signals in a Radio System,xe2x80x9d to Richards, et al, (issued Aug. 29, 2000), and commonly-owned U.S. Pat. No. 6,483,461 xe2x80x9cApparatus and Method for Locating Objects in a Three Dimensional Space,xe2x80x9d (issued Nov. 19, 2002) teach various methods to employ a network of fixed reference radios to find the positions of one or more mobile impulse radios. Positioning methods taught therein include multi-lateration using multiple signal time-of-arrival (TOA) computations among the various radios, coupled with angle information calculated using differential time-of-arrival (DTOA) from at least two reference radios in the network. A limitation common among these inventions is the requirement for multiple radios to determine the position of another.
Single radio implementations related to positioning exist. Impulse radios have been used to measure distance of other radios by analyzing the free space signal propagation loss as discussed in commonly owned an co-pending application Ser. No. 09/537,263 xe2x80x9cSystem and Method of Estimating Separation Distance Between Impulse Radios Using Impulse Signal Amplitudexe2x80x9d (filed Mar. 29, 2000). However, without relative angle information or distance information from another source, the position of the opposing radio cannot be determined. Additionally, U.S. Pat. No. 6,133,876, xe2x80x9cSystem and Method for Position Determination By Impulse Radio,xe2x80x9d to Fullerton, et al, (issued Oct. 17, 2000), describes using multiple impulse radio transceivers to determine relative distances of said transceivers with respect to each other and deriving each transceiver""s position in terms of Cartesian coordinates via multi-lateration. The patent also teaches use of one transceiver to position another, however, the transceiver uses TOA ranging and must employ a direction finding antenna in addition to the ultra wideband antennae transmitting and receiving the impulse signals.
There are technologies that employ single units to determine angular position of objects. For example, U.S. Pat. No. 4,017,854, xe2x80x9cApparatus for Angular Measurement and Beam Forming with Baseband Radar Systems,xe2x80x9d to Ross (issued Apr. 12, 1977), describes a pulsed radar system comprised of two separated receiving antennae, said system being capable of estimating the relative angle of an illuminated object using DTOA. However, baseband systems, compared to impulse systems, use much greater transmit power and are not very well channelized rendering such systems very susceptible to interference. Thus, they tend to be impractical for applications other than in electromagnetically clear space or in areas in which interference with other systems is not a concern.
Hence, there exists a need in the art for a single unit that may estimate the location of an RF emitter. In particular, there exists a need in the art for a single apparatus capable of approximating the position of an impulse radio transmitter.
An impulse radio receiver capable of determining angular offset, and thus position, of a transmitting impulse radio includes two antennae disposed within the receiver such that they are separated by some distance. Both antennae are coupled to an impulse radio, preferably utilizing a multiple correlator design. One version of the invention incorporates a cable delay into one of the antenna-radio couplings.
The transmitted signal is received by the impulse radio in the form of two pulses, one from each antenna. One pulse may be delayed in time with respect to the other, by virtue of the cable delay for embodiments employing this structure, and by virtue of the transmitter geometry with respect to the receiver creating a longer time of flight for the signal received at the opposite antenna. The delay of one signal with respect to the other is measured and the angle of offset may be approximated.
Time delay may be measured by a variety of ways in this system. Such techniques include autocorrelating the composite waveform and measuring the distance between correlation output peaks in the time domain. Additionally, using pulse leading edge detection algorithms, the leading edges of the pulse may be found and the delay between the leading edges may be measured directly. Various combinations of both techniques are also employed. Another embodiment describes using two synchronized impulse radios and employing the same techniques.
Those skilled in the art will appreciate that the design may incorporate three or more antennae similarly coupled to the receiving radio with cable delays. A three-antennae version would yield two forms of offset angle information depending on the configuration: (1) two offset angles in the same plane; or (2) offset angles in two dimensions. This information will facilitate more accurate positioning of the transmitter in two dimensions or estimation of transmitter position in three dimensions respectively. Addition of a fourth antenna would enable three-dimensional positioning.
Because of the high pulse rate and improved data processing over prior systems angular error is much improved allowing for closer spacing within the antennae array. In fact, another version teaches maintaining scanning receiver locks on the arriving pulses thereby allowing position information to update at the bit rate. This is desirable for applications requiring position information for fast moving objects.